Does anyone know more details about how the O2 sensor heater wires coming from the MAS relay work? Specifically, is it strictly key on, apply power to these wires? Or does the ECU cut power to the heater wires if the sensor is warmed up enough?

I'm trying to determine if this source of power is good enough for a wideband O2 controller. I don't really feel like hooking into another area for power, but I will go cigarette lighter if the original O2 heater from the MAS relay is not a reliable 12V switched power source.

I guess it's not an easy question to answer, not many people have fully characterized the behavior of the CIS computer.

But I was looking at the wiring diagram for the circuit I'm interested in:



A thought occurred to me to make a modified MAS relay and power the O2 heater circuit off the fuel pump circuit. The diagram isn't too clear on this but it seems like pin 2 on the MAS relay is a signal (not power) for the fuel pumps to turn on. I could probably remove pin 18 from the CIS computer and connect pin 2 here. This way, the O2 heater circuit is always on if the fuel pump is on.



Edit: Fuel pump gets cut at redline, probably bad idea. Maybe better to use pin 21 as a power source. It's already fused.

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I used the fuel pump circuit as I had removed the original ECU. I'm not sure the cutoff at redline is a huge deal. I assume you're running some sort of aftermarket ecu?

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

Yeah I'll be running an aftermarket ECU, but I want to keep the stock computer in place and happy. The only thing I am taking away from the stock ECU is the fuel control, but I still want everything else like functional AC and no CEL thrown.

For future reference incase anyone is looking for the same information, it looks like the CIS computer cuts power to the O2 heater circuit under load. I drove around with the multimeter hooked up to the wires and was able to characterize it's behavior.

On a cold start, the O2 heater circuit does not get power for about 15 seconds. I don't have an accurate time for this because I wasn't expecting this to happen and didn't time it. I'm guessing this action is to purge the condensation so the O2 sensor doesn't get destroyed from starting the engine.

Under heavy load conditions, even before reaching operating temperatures, the O2 heater circuit is cut. It's not RPM based since sitting idle and revving the engine slowly keeps the heater circuit alive. If I floor the gas pedal in 2nd gear idling, the heater circuit gets cut even at 2000 RPM.

So to answer my own question, I'll need to find a new power source for the wideband O2 sensor. I'll either modify a MAS relay if I can easily figure out what to do in there, or just run new wires from the cigarette lighter.

This isn't really relevant to my original post, but I just wanted to share some success! A wideband O2 sensor with a controller that outputs a narrowband signal can work with the stock CIS computer.



I just finished installing an Innovate LC-2 controller with a Bosch LSU 4.9 sensor and the stock computer seems to be ok with the signal that it outputs on Analog Output 2.



I was able to connect the controller to my computer and check out the AFR while idling and revving in my garage. I haven't taken the car outside to monitor AFR under WOT to see how CIS behaves yet. But playing with the throttle in neutral shows me that the CIS computer corrects to 14.7 pretty damn fast.

In the place of the O2 sensor heater, I connected a 150 ohm 5W tolerant resistor, wrapped it up in black vinyl tape, and left it out in the transmission tunnel.





I do plan to later connect the more useful wideband signal to a Microsquirt ECU, but my narrowband O2 sensor was already on it's way out (threw code 17 at me the other day, cleared it never came back) and this was work that had to be done anyway. If the narrowband signal wasn't accepted by the stock computer, I would have just left the wiring from the wideband controller in place and replaced the O2 sensor with another one for the time being.

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